Because he is stupid. He was not a "news cameraman" but a leftist agitator who did some freelance work for NBC. They hired him exactly one day before he felt feverish and two days before he was diagnosed. Then the leftist "doctor" Snyderman left the country with her crew.
The relatives in the Dallas apartment, especially Duncan's girlfriend, show no signs even after being exposed to a very contagious victim. Too many questions.
No easy answers, but obviously Duncan was not as contagious. Also he may have been careful to wash up after vomiting, etc around the apartment. His "stepdaughter" disinfected the place days before Texas bothered to send the "cleaning guys" over there.
Bradley handled a sick person with no protection before she was diagnosed.
If you are around a dying victim or their splatter, you will get infected. Before they are dying, earlier stages of fever etc, you will probably get it from contact or sneezes. Sneezing is not a normal symptom.
Interesting points, especially about the later stages of the disease and the contagious rate.